A distance measuring device called a LIDAR (Light Detection and Ranging; Laser Imaging Detection and Ranging) is known. With this device, a measured object is irradiated with pulse light from a light source and scattered light scattered with the measured object is detected by an optical detector. Turnaround time of the light is longer as a distance from the distance measuring device to measured object, that is, an optical path length is longer. Therefore, the distance can be measured by using a time difference between a time point when the light source emits the light and a time point when the scattered light is detected. Furthermore, a distance image of the measured object can be generated by scanning the pulse light of the measured object to measure the distances to positions on the measured object.
Power of the light emitted from the light source is approximately a product of the number of points to be measured (number of points in one frame), power of the light emitted to single point, and a frequency of measurement (frame rate). In order to secure safety, power of light emitted from a light source is restricted. Therefore, these three cannot be made large simultaneously.
When a moving measured object is measured, for example, a frame rate not less than 10 fps is needed. Moreover, when a measured object in a long distance is measured, light strong in power is needed to be emitted for single point. This causes a less number of points to be measured and a distance image in high resolution cannot be obtained. On the other hand, in the case of a greater number of points to be measured, the frame rate is needed to be lowered and measurement for a long distance is needed to be given up.